This invention relates to electrically heated windows and the like. More particularly, it relates to an electrically heatable transparent sheet comprising an electrically conductive thin layer with a surface resistance of 1 to 10 ohms per square surface which is applied to a transparent substrate and connected to current-bearing electrodes.
Various types of electrically heatable windows are known whose heating resistor is a transparent thin resistive layer. They are, for example, used as sight plates to prevent the impairment of transparency by moisture precipitation. The transparent thin resistive layer may be made from a wide variety of materials. Typically metallic layers, particularly of gold and silver, are used; but layers are also used which are based on metallic compounds, particularly layers of tin oxide or indium oxide or corresponding mixed layers. The layers can be produced by vacuum evaporation or sputtering of the corresponding metals in vacuum, by spraying of solutions of the metallic compounds and subsequent pyrolytic dissociation and also by ion diffusion. They can furthermore be applied either directly to a surface of a silicate glass plate or to another transparent substrate, such as a transparent plastic film, which is combined with one or several silicate glass plates in a laminated glass plate. All these various types belong to the state of the art.
Frequently the current density in extremely thin resistance layers is very high, so that these layers are frequently operated close to their capacity. The current density is particularly high at points where inhomogeneities exist within the resistive layers. Such inhomogeneities can rapidly lead to a local overloading of the resistive layers by current peaks. Such an inhomogeneity is also represented by the abrupt transition from the relatively low-resistance electrodes to the relatively high-resistance resistive layer. Consequently the points of transition from the electrodes to the thin resistive layers are frequently the critical points in such heated plates where locally restricted current peaks are easily generated. At these points the resistive layer may char and lose its conductivity. If this happens, other points must then carry even more current, with the result that they become overloaded and also char. In the most unfavorable case, a charred line finally is generated all along the interface between the resistive layer and the electrode which interrupts current altogether.